G Model
CCLET 3521 1–7
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T. Zhang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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[16–21]. To achieve bipolarity in the molecular designs based on
carbazole, various moieties capable of accepting electrons were
incorporated to give novel bipolar hosts [22–24]. For example,
Yang et al. reported a bipolar dendrimer with carbazole units as
hole-transporting units and oxadiazole units as electron-trans-
porting units with maximum efficiencies of 16.8 cd Aꢀ1 and 5.7%
obtained for solution-processed green PHOLEDs by using Ir(ppy)3
as guests [23]. Most recently, Li et al. reported a carbazole-based
molecule as a universal bipolar host material by attaching 3,6-
bis(3,6-di-tert-butyl-carbazol-9-yl)-carbazole and pyrazole to the
dimethylbiphenyl core, achieving high luminance efficiencies of
35 cd Aꢀ1 and 39 cd Aꢀ1 for green and red electrophosphorescence,
respectively, by incorporating electron-transporting 2,20-(1,3-
phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7)
in the emitting layer as a mixed host [24].
In this paper, we report the synthesis and characterization of two
novel carbazole-based dendrimers, namely G1SF and G2SF, for
potential application as bipolar host materials. The electron-
accepting dibenzothiophene was selected as the core, and the
well-known electron-donating oligo-carbazole dendrons were
attached at both terminals of the dibenzothiophene. Furthermore,
tert-butyl groups are introduced into the dendrimers surface to
enhancethesolubilityofthesedendrimers.Thedendrimersdesigned
in this way are observed to exhibit excellent thermal stability, to
possess relatively shallow, highest occupied, molecular orbital
(HOMO) levels, and to have the spatial separation of the HOMO
andLUMO energy levels. The green PHOLEDsusingG1SF or G2SF,asa
host material by the spin-coating method withtraditional Ir(ppy)3 as
chlorobenzene were filtered through a 0.45
m
m PTFE filter and
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spin-coated on PEDOT/PSS film, the thickness of which was
controlled at 40 nm by adjusting the spin rate. The substrate was
transferred into a vacuum chamber to deposit the TPBI layer at a
base pressure less than 10ꢀ6 Torr. Finally, the device fabrication
was completed through thermal deposition of LiF (1 nm) and then
capping with Al metal (100 nm) as cathode. The EL spectra, CIE
coordinates, and current density–voltage–luminance relationships
of devices were measured by the JY SPEX CCD 3000 photometer
and a Keithley 237 instrument. All the measurements were carried
out in ambient condition at r.t.
2.2. Synthesis and characterization
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The carbazole dendrons D1 and D2 were synthesized and
characterized according to literature methods [27].
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Synthesis of compound 1: To a solution of dibenzothiophene
(940 mg, 5.5 mmol), iodine (1.34 g, 5.2 mmol), iodic acid (1.58 g,
9.0 mmol), acetic acid (5 mL) and chloroform (5 mL), was added
water (0.5 mL) and concentrated sulfuric acid (0.5 mL, 98%). The
mixture was stirred at 50 8C for 18 h, and then 10 mL water was
added. The organic layer was separated and washed with diluted
HCl and brine, then dried over anhydrous MgSO4. The solvent was
removed under vacuum and the residue was recrystallized from a
mixed solvent of chloroform/ethanol (v/v = 3:1) to give 1 as a light-
yellow crystal (1.83 g, 75% yield). EI-TOF-MS (m/z): 435.84 [M+].
Synthesis of G1SF:
A mixture of compound 1 (100 mg,
0.23 mmol), D1 (154 mg, 0.55 mmol), CuI (5 mg, 0.03 mol), and
K2CO3 (20 mg, 0.15 mmol) were added to a 50 mL 2-neck flask, and
then 18-crown-6 (5 mg, 0.02 mmol) and o-dichlorobenzene
(20 mL) were added under a nitrogen atmosphere. After stirring
for 12 h at 160 8C, the reaction mixture was cooled to r.t. The
solvent was removed under reduced pressure, dichloromethane
and water were added. The organic layer was separated and
washed with diluted HCl and brine, then dried over anhydrous
MgSO4. The solvent was removed to dryness and the residue was
purified by column chromatography over silica gel with petroleum
ether/dichloromethane (4:1) as the eluent to give G1SF as a white
solid (224 mg, 60% yield). Confirmation data: 1H NMR (400 MHz,
doped emitter, show the maximum luminance efficiency (hL) of
19.83 cd Aꢀ1 and a maximum external quantum efficiency of 5.85%.
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2. Experimental
2.1. Materials and methods
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Chemicals, reagents and solvents from commercial sources are
of analytical, or spectroscopy grade and used as received without
further purification. The 1H NMR spectra were recorded on an
AVANCE-400 NMR spectrometer (400 MHz). Mass spectra were
recorded on a gas chromatograph-time of flight (GC-TOF) mass
spectrometer (Micromass, UK) for EI-TOF-MS and a MALDI micro
MX (Waters, USA) for matrix-assisted laser desorption time of
flight (MALDI-TOF) mass spectra. Thermogravimetry analyses
(TGA) were carried out using a Pyris1 TGA (PerkinElmer Corp., USA)
at a heating rate of 10 8C minꢀ1 under a nitrogen atmosphere. The
fluorescence and UV–vis absorption spectra measurements were
performed on a Hitachi F-4600 spectrofluorophotometer and a UV-
265 spectrophotometer, respectively. Electrochemical measure-
CDCl3): d 8.28 (d, 2H, ArH), 8.15 (d, 4H, ArH), 8.10 (s, 1H, ArH), 8.08
(s, 1H, ArH), 7.71–7.69 (m, 2H, ArH), 7.46–7.43 (m, 4H, ArH), 7.37
(s, 2H, ArH), 7.34 (s, 2H, ArH), 1.45 (s, 36H, CH3); MALDI-TOF-MS
(m/z): calcd. for C52H54N2S, 738.4008; found, 738.3795 [M+].
Synthesis of G2SF: A mixture of compound 1 (100 mg,
0.23 mmol), D2 (400 mg, 0.55 mmol), CuI (5 mg, 0.03 mmol),
and K2CO3 (20 mg, 0.15 mmol) were added to a 50 mL 2-neck flask,
and then 1,10-phenanthroline monohydrate (10 mg, 0.05 mmol)
and toluene (20 mL) were added under a nitrogen atmosphere.
After stirring for 48 h at 110 8C, the solvent was removed under
reduced pressure. Water was added and the mixture was extracted
with dichloromethane. The organic layer was separated and
washed with diluted HCl and brine, then dried over anhydrous
MgSO4. The solvent was removed to dryness and the residue was
purified by column chromatography over silica gel with petroleum
ether/dichloromethane (4:1) as the eluent to give G2SF as a white
solid (58 mg, 35% yield). Confirmation data: 1H NMR (400 MHz,
ments were made by using a conventional three-electrode
configuration and an EG & G PAR 283 potentiostatic instrument
at a scan rate of 100 mV sꢀ1. A glassy carbon working electrode, a
Pt-wire counter electrode, and a saturated calomel electrode (SCE)
as reference electrode were used. All measurements were made at
r.t. on samples dissolved in dichloromethane, with 0.1 mol Lꢀ1
tetra-n-butyl ammonium hexafluorophosphate (Bu4NPF6) as the
electrolyte, and ferrocene as the internal standard [25]. Density
functional theory (DFT) calculations using B3LYP functional were
performed. The basis set used for C, H, N atoms was 6-31G. There
are no imaginary frequencies for both optimized structures. All
these calculations were performed with Gaussian 09 [26].
The patterned ITO substrates were cleaned by successive
ultrasonications in detergent, deionized water, ethanol, and
dichloromethane, followed by treatment with UV-ozone for
20 min. A 40 nm thick PEDOT:PSS film was first spin-coated on
pre-treated ITO substrates from an aqueous dispersion and baked
at 120 8C for 40 min in air. Subsequently, blends of host: Ir(ppy)3 in
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CDCl3):
d 8.56 (s, 2H, ArH), 8.29–8.24 (m, 6H, ArH), 8.13 (s, 8H,
ArH), 7.92–7.89 (m, 2H, ArH), 7.64–7.57 (m, 8H, ArH), 7.43–7.41
(m, 8H, ArH), 7.32 (d, 8H, ArH), 1.45 (s, 72H, CH3); MALDI-TOF-MS
(m/z): calcd. for C116H114N6S, 1624.2516; found, 1624.0037 [M+].
3. Results and discussion
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The synthetic routes for the target dendrimers (G1SF, G2SF) are
shown in Scheme 1. First, the important intermediates including
the carbazole dendrons (D1, D2) were synthesized according to
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Please cite this article in press as: T. Zhang, et al., Synthesis and characterization of carbazole-based dendrimers as bipolar host